March 1992
THINKING LIKE AN ENGINEER
In the past few years, I have been interested in reading literature outside the podiatric profession.
Some of the most enlightening information comes from the engineering profession.
Structural engineers spend their careers analyzing loading forces on structures such as buildings and bridges and how these loading forces are converted into tensile, compression or torsional forces within the components of the structure. It is the detailed analyses of these internal forces within the components of structures which allow engineers to determine the type and size of the structural components necessary to prevent structural failure.
For engineers, it is relatively easy for them to improve their designs of structures by performing tests on the component materials of the structure to determine the failure points of each specific material under increasing load.
For example, engineers know the exact loading force which is required to break or bend a beam of wood or steel.
They also know the tensile force at which a steel cable will at first reach its elastic limit and then, with increasing tensile force, the force at which the cable will break.
Another important method which an engineer can use to determine whether a structure will or will not fail under its expected loading forces is to build a model of the structure and analyze its response to various loading conditions.
This "trial and error" method of structural testing is very important since even the most sophisticated computer analysis of a structure may have faults in it due to the inherent complexity of the interaction of the components with each other.
As podiatrists, we could all benefit from thinking like structural engineers at times. Unfortunately, for many of us, our podiatric biomechanics education instructed us to simply measure externally apparent deformities of the foot and lower extremities (i.e. tibial varum, rearfoot varus, forefoot to rearfoot relationship) to determine the correct orthosis prescription for the patient.
Our podiatric biomechanics education involved very little instruction in regards to the internal tensile, compression and torsional forces which occur within the structural components of the human foot and lower extremity.
I believe that analysis of externally measurable deformities of the foot and lower extremity does not give us near enough information to predict the mechanical behavior of the foot and lower extremities during weightbearing activities and therefore is insufficient to prescribe the best orthoses for our patients.
As mentioned in my last newsletter in which I discussed the internal compression and distraction forces within the midfoot, weightbearing loads on the foot cause compression forces between the dorsal joint surfaces of the bones of the midfoot and tensile forces in the plantar ligaments and plantar fascia.
The magnitude and exact location of pathological tensile, compression or torsional forces on the internal structures are very difficult to predict using the externally measurable parameters such as tibial varum, rearfoot varus, and forefoot to rearfoot relationship.
In other words, it would be very hard to predict which bone, joint, ligament, tendon or muscle in the foot would become painful just by doing the standard biomechanical examination.
If podiatric biomechanics could somehow give us enough information so that we could predict, in each type of foot, which specific structure of the foot or lower extremity would become painful during weightbearing activities, then we would also know the best way to treat these painful conditions with foot orthoses or shoe modifications.
I feel we are a very long way from attaining these goals since we are unable to do enough "material tests" on the bones, ligaments, tendons or muscles in feet.
In other words, we don't have any simple way of determining the exact size, strength and shape of the bones, ligaments, tendons and muscles of each person's feet which would give us a better idea of their failure point.
However, what we do have is a fairly good knowledge of normal and abnormal gait cycles (i.e. through video analysis), the sequential activity of the lower extremity muscles during gait (i.e. through electromyography), the structure of the foot (i.e. through detailed anatomic dissections) and the magnitude and direction of the ground reaction forces acting on the plantar surface of the foot during gait (i.e. through force plate analysis).
Using these known variables, we can create basic models of the foot and lower extremities.
With these crude models, an intelligent prediction can be made whether one of the structural components (i.e. bone, ligament, joint, tendon or muscle) of the foot is under tensile, compression or torsional loading stresses during gait.
It is with this knowledge and the experimental observations made by a few researchers in the past 50 years that we can be very certain of the various types of stresses which the structural components of the foot and lower extremity are subjected to during different weight bearing activities.
And once we are certain of the various stresses on the structural components then we can design our mechanical therapy specifically to reduce or eliminate those stresses.
It is in this way that podiatrist, by thinking like engineers, can improve our decision making processes so that our patients will benefit by more specific and efficient orthosis prescription and design.
[Reprinted with permission from: Kirby KA.:
Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters.
Precision Intricast, Inc., Payson, Arizona, 1997, pp. 267-268.]
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